1. Field of the Invention
The present invention relates to a process for producing a propylene-ethylene block copolymer. More particularly, it relates to an improved process for producing a hetero-block copolymer in which an ethylene-propylene random copolymer is bonded to a polypropylene main chain.
2. Description of the Prior Art
It has been known to obtain a hetero-block copolymer by two stage polymerizations in which propylene is polymerized in a diluent such as an inert liquid hydrocarbon in the presence of a catalyst comprising titanium trichloride and an organoaluminum compound as Ziegler-Natta catalyst in the first stage and then, ethylene and propylene are random-copolymerized in the presence of the propylene homopolymer in the second stage.
The titanium trichlorides used by these conventional processes have been obtained by reducing titanium tetrachloride with an organoaluminum compound or a metallic aluminum or further heat-treating and pulverizing the product, and they are especially the products referred as TiCl.sub.3 -AA. When these titanium trichlorides are used, polymerization activities are not satisfactory and contents of non-crystalline polymers as by-products are much. It is not possible to provide satisfactorily high productivity per a catalyst because of the low polymerization activity. Therefore, it is necessary to separate catalyst residues from the polymer by dissolving the catalyst residues into a diluent by treating a slurry of the polymer with an alcohol etc. and separating the polymer from the diluent after the polymerization. The non-crystalline polymers as the by-products are preferably removed from the polymer product because high rigidity as one of important characteristics of polypropylene is lowered by the non-crystalline polymers. An amount of the non-crystalline polymers increases in a copolymerization of propylene-ethylene block copolymer. This causes to decrease a yield of the object polymer and also to adhere a rubber-like polymer on an inner wall of a reactor and to aggregate particles of the polymer and to clog pipes etc., whereby a stable continuous operation is prevented.
Thus, it further requires processes for separating and recovering the alcohol used for the separation of the catalyst residues and the non-crystalline polymers from the diluent to increase costs for producing the object polymer.
The advantageous physical characteristics of the block copolymers are to have remarkably improved impact strength and brittleness at low temperature without substantial decrease of high rigidity which is the advantageous characteristic of a propylene homopolymer. As shown in Japanese Examined Patent Publication No. 31119/1972, the impact strength is improved depending upon an increase of a ratio of propylene to ethylene in the second stage, however, a formation of the non-crystalline polymer is also increased. Only 80 to 90% of the object block copolymers has been recovered from the diluent solution of the polymers.
It has been proposed to decrease a formation of the non-crystalline polymers by polymerizations to produce a block copolymer in which a crystalline propylene polymer is produced at a ratio of more than 90% by weight based on the total polymers in the first stage as disclosed in Japanese Examined Patent Publication No. 1836/1964. Such process for forming ethylene-propylene random copolymer at a relatively low ratio in the second stage causes inferior effect for improving the impact strength as one of the important characteristics of the block copolymer.
It has been proposed to increase concentrations of monomers, that is, to use liquid propylene as the monomer instead of an inert liquid hydrocarbon as a diluent as one process for increasing a productivity of the polymer per the catalyst. In accordance with such process, the productivity of the polymer per the catalyst can be increased, however, it is not enough high because the polymerization activities are not so high when the conventional titanium trichloride which is obtained by reducing titanium tetrachloride with a metallic aluminum and which has a formula of TiCl.sub.3.1/3AlCl.sub.3 so called TiCl.sub.3 -AA is used. As it is found in Japanese Examined Patent Publication No. 32414/1971, the productivity of the polymer is only about 5 kg. per 1 g. of titanium trichloride.
Moreover, it is difficult to increase a content of ethylene component in the ethylene-propylene random copolymer since the random copolymerization of ethylene and propylene is carried out in the liquid propylene as one of the comonomers. Various processes for increasing the content of ethylene component have been proposed. For example, in Japanese Examined Patent Publication No. 10116/1978, a random copolymerization of ethylene and propylene is carried out at a low temperature of -45.6.degree. to 23.9.degree. C. (-50.degree. to 75.degree. F.) in the second stage, to increase a content of ethylene component. When the copolymerization is carried out at such low temperature, a polymerization velocity is remarkably low and a special cooling apparatus for maintaining the reactor at such low temperature is required. This is remarkably disadvantageous for an industrial operation.
In Japanese Examined Patent Publication No. 32414/1971, the vapor phase is discharged from the reactor and then, a new vapor phase is fed into the reactor in the random copolymerization of ethylene and propylene in the second stage to increase the content of ethylene component. In this process a compressor for recycling the vapor phase is needed.
The inventors have studied to develop an advantageous industrial process for a block-copolymerization in liquid propylene in a main stage to provide a large content of ethylene component in the ethylene-propylene random copolymer formed in the second stage.